There's a lag between oil prices and petrol prices. When you buy...

There's a lag between oil prices and petrol prices.

When you buy petrol the price you pay is much more representative of the price of oil at the time that oil was originally sold (as crude oil from the primary supplier) than the price of oil at the time of final product purchase.

You'll also see the price of petrol falling when tbe price of oil rises. You need to look back at the oil price at T-(lag period).